Add configuration for Travis CI

This pull request adds configuration for Travis CI, a continuous integration service available to open source projects for free. It has many benefits, but a few of my favorites are testing against multiple ruby versions, automatic testing of pull requests, and code coverage statistics when combined with coveralls (also free for open source)

In preparation for this pull request, you'll need to create a travis-ci.org account with the github user who owns this repo. I don't know the exact steps, but it should be fairly straight forward. https://travis-ci.org/

I would suggest setting up the account first and enabling the github hook for this repo prior to merging the pull request, that way when the PR is merged, it automatically kicks of the first build test.

At present, the builds will fail, however the failure that Travis encounters is corrected in pull request #31. If that is merged first, I don't think Travis should fail. If it is merged later, you should see the builds succeed after. You can see an example of the failing Travis build here: https://travis-ci.org/tdg5/beaneater/builds/11410087

I hope you won't find this PR to be forward; I think Travis is a valuable tool that can really enhance testing.

If you have any questions or if I can help with any part of this, please let me know!